1. Field of the Invention
The present disclosure relates to an ion mobility spectrometer detection method using dopants.
2. Description of the Related Art
Ion mobility spectrometry (IMS) has been proposed and developed as a trace detection technique since the 1960s. Earlier patents (U.S. Pat. Nos. 3,699,333 and 4,777,363) described the gist of this technique in details. The operation of an ion mobility spectrometer is based on the following principles: sample vapor or vaporized solid sample particles to be detected are ionized to form ions which then migrate directedly through a drift tube under the influence of a weak electric field. A collector provided at an end of the drift tube is used to measure the drift times taken by the ions to pass through the electric field, so that mobilities of the ions i.e., drift velocities of the ions under the influence of per unit electric field intensity, can be calculated from the drift times taken by the ions. Since the mobilities of ions obtained from various substances are specific under certain conditions depending upon the mass the number of charges and the space structure of the ions, the types of the substances can be determined by matching the detected mobility values of the resultant ions with those collected in a standard library.
The drift tube is the core component of an ion mobility spectrometer, the basic structure of which is shown in FIG. 1, comprising a sample inlet, an ionization region, an ion gate, a drift region and a collector etc.
At the forefront of the drift tube is provided the sample inlet into which the sample to be detected is introduced and carried by a carrier gas to the ionization region. Purified air is usually used as the carrier gas. Electrons emitted from an ion source 63Ni in the ionization region will react with N2 or O2 comprising the air and a small amount of water molecules present in the air to generate reactant cluster ions, such as (H2O)nH+, (H2O)nNO+, (H2O)nNH4+, (H2O)nO2−, (H2O)n(CO2)mO2−, (H2O)nOH+, etc. When entering into the ionization chamber, molecules of a substance to be detected or a sample will react with the reactant ions to form product ions. When the ion gate is opened, the product ions enter into the drift region from the ionization region, migrate directedly under the influence of the electric field, and finally impinge onto the collector disposed at the end of the drift region opposite to the ionization chamber. In this way, weak current signals representing the strength of ion flows are generated, and the resultant spectrum shows real-time information indicating variation of ion strength with time. Since the mobilities of various product ions are different from each other, the drift times required to reach the collector also vary from each other. By analyzing the spectra and matching the spectra with those collected in the standard library, the type of the substance can be determined.
The ionization of gas molecules of the sample to be detected, which occurs in the ionization region of the drift tube, is regarded as a process of secondary ionization. The concentration ratio of the carrier gas to the sample vapor causes the molecules of the carrier gas to be more easily ionized by the ion source than the molecules of the sample vapor. Therefore, air molecules first undergo ionization to form reactant ions. Since the free path for the ionized molecules of the carrier gas is much shorter than the geometrical size of the ionization chamber, the ionized molecules of the carrier gas will collide with the sample vapor molecules frequently, thereby leading to the transfer of electric charges or protons from the ionized molecules of the carrier gas to the sample molecules. The occurrence of such transfer reaction of electric charges or protons depends on proton or electron affinity of the molecules participating in the reaction. The electric charges will be transferred from the molecules having small electron or proton affinities to those having large electron or proton affinities.
For an ion mobility spectrometer in actual use, the addition of dopants is usually adopted to modify molecule composition of the reactants and the ionization mechanisms so that chemical composition of the resultant product ions can be changed to improve the sensitivity and selectivity of the instrument, It is necessary that the dopant molecules have an electron affinity less than that of the sample molecules (e.g., explosives) and higher than that of other components contained in the carrier gas. Thus, cluster ions having relatively stable composition can be preferably generated through ionization to prevent interferences with lower affinities in the carrier gas from participating in the ionization reaction. Meanwhile, since the electron or proton affinity of the sample molecules is greater than that of the dopant molecules, these cluster ions then react with the sample vapor molecules to form the sample molecule ions for detection. The addition of dopants can cause a shift in peak positions corresponding to the resultant product ions in the spectrum. In this way, the ion peaks, which can be hardly distinguished due to peak overlap in a case where no dopant is added, can be separated from each other to realize identification of the target components even in the presence of interferents. The currently common-used dopants for the detection of explosives are halogenated compounds, and the dopants usually used for drug detection include nicotinamide, acetone, ammonia water or the like.
Over the past thirty years, a large amount of research on IMS techniques has been made in the developed countries, and a great number of patents, such as U.S. Pat. No. 4,311,669, U.S. Pat. No. 4,551,624, DE Pat. No, 19502674, WO Pat. No. 9306476, etc., involving structural design, separation principle, and sampling techniques have been issued. Some of these patents describe the methods of adding dopants and the associated applications.
PCT patents, for example, WO Pat. No. 2006129110 and WO Pat. No. 2004102611 discloses ways of adding chemical dopants. The ion mobility spectrometer disclosed in PCT patent WO Pat. No. 2006129101 employs at least two reservoirs for supplying various dopants. The reservoirs are connected with an ionization chamber of the instrument. An inlet port is provided at a side of a selectively permeable membrane facing a sample inlet. Thus, the sample gas is brought into contact with the dopants before being ionized. The circulation gas passage in the drift tube and the doping gas passage are separated from each other. The system disclosed in PCT patent WO Pat. No. 2004102611 comprises a molecular sieve doped with a dopant which can be used to continuously supply a first dopant. The system further comprises additional reservoirs containing various dopants to selectively supply other dopants in addition to the first dopant into the air by means of switches.
Furthermore, the apparatus disclosed in U.S. Pat. No. 6,495,824 comprises a plurality of reservoirs containing various dopants to selectively add different dopants into the flow of the carrier gas according to changes in detection signals. The added dopants react with samples to generate resultant adduct products having different mobilities. An information library, containing known reaction information of an substance to be detected with different dopants, can be established. By comparing between the observation results and the data in the information library for specific combination of the sample and various dopants, it can be determined whether the sample contains the substance to be detected. In patent WO/2007/082941, it is disclosed that a substance to be analysed is injected by means of an atmospheric pressure ionization interface at the inlet of the instrument and meanwhile an additive is introduced by adding it to the nebulizing gas. A doping source material is combined with a material for drying and cleaning the circulation gases/vapors as is shown in U.S. Pat. No. 2002088936. In U.S. Pat. No. 5,491,337, it is disclosed that a low concentration of dopant is mixed with a carrier gas in an enclosed container disposed in front of a sample gas inlet of the instrument, and then is introduced, together with the sample gas, into the ionization chamber. As disclosed in In EP Pat. No. 1672363, the sample gas is mixed with the doping gas before entering into the instrument, or the doping gas is added into the drift gas, so as to eliminate the problem caused by interference when the ion mobility spectrometer is used to analyze a large number of inert gas samples.
Patents WO2006/123107, EP 0509171, U.S. Pat. No. 5,283,199, U.S. Pat. No. 5,234,838, U.S. Pat. No. 5,032,721, DE 19609582, DE 10212110, WO2007/085898, etc., describe various dopants used for analysis by ion mobility spectrometry to adapt to different applications. A dopant containing diamyl ketone, for example, is added into a circulation gas passage. A small amount of a dopant sulphur dioxide, for example, is added into a sample to be detected by means of a temperature-controlled permeable tube. Acetone and carbon tetrachloride, for example, can be added as dopants into the carrier gas before the sample is injected. A small amount of substituted phenol dopants (e.g., methyl salicylate, 2-hydroxy hypnone) and amine dopants (e.g., methylamine), for example, are added into a sample to be detected by means of a temperature-controlled permeable tube. The content of ammonia gas in gas mixtures, for example, is monitored by using dimethyl methylphosphonate (DMMP) as the dopant. Amides, for example, are supplied as ionization dopants to generate reactant ions and then interact with a substance to be detected for the detection of peroxide explosives.
FIG. 2 depicts a schematic view of the structure of an ion mobility spectrometer system using a dopant in the prior art.
The detection system comprises a sample feeding port 21, a drift tube 20 containing an ion source 10, and a gas passage system communicated with the drift tube. A dopant gas source 41 is used for supplying a dopant gas that is added into the system by the carrier gas or the drift gas. A sampling substrate is used for collecting a certain amount of a sample and is guided, together with the collected sample, into the sample feeding port of the drift tube. Thereinto, the gas passage system comprises a pump (not shown) and a filter 34. Outlet of the gas passage system is communicated with a gas inlet 31 disposed at an end of a drift region opposite to an ionization region and a gas inlet 32 disposed at a position in the ionization region which is adjacent to an ion source, respectively, to supply a clean gas flow used as the drift gas and the sample carrier gas, such as air. An inlet of the gas passage system is communicated with a gas outlet 33 disposed at a position in the ionization region which is adjacent to an ion gate, to guide the unionized drift gas and carrier gas molecules out of the drift tube. Then, the unionized drift gas and carrier gas molecules are dried and purified to be used for circulation in the gas passage system. Arrangements of the dopant gas source 41 in the gas passage, i.e., ways of adding the dopant, can be varied. The dopant gas source 41 can be arranged in the carrier gas passage connected to the gas inlet 31 or in the drift gas passage connected to the gas inlet 32. The obtained dopant vapor is mixed with the drift gas or the carrier gas to enter into the drift tube. Alternatively, the dopant gas source 41 can be combined with a drying and cleaning device and also can be connected with a sample feeding device (not shown). Usually, the sample feeding port and the ionization region are separated from each other by a selectively permeable membrane. A doping inlet is provided at a side of the selectively permeable membrane facing the sample feeding port, so that the sample gas is brought into contact with the dopant before ionization.
In the prior art, in addition to a gas source made of semi-permeable membrane to contain a dopant, it is also necessary to control the temperature of the gas source to supply a certain amount of the dopant maintaining a proper concentration. Thus, the gas passage structure and the equipment design become complicated. Furthermore, since the dopant is continuously supplied by a single gas passage, it's difficult to realize the rapid and accurate control of the amount of the dopant, which is easily caused to be either over high due to the accumulation of the dopant in the system or over low due to the depletion or agglomeration of the dopant in its source, thereby affecting the sensitivity of the detection. Additionally, since most of the dopants are corrosive substances, the dopant gas source, the gas passage and associated accessories need to be made of corrosion-resistant material. This undoubtedly increases the manufacturing and maintaining cost of the detection system.